Aaron texts the wrong number and they didn't know. They sexted who they thought was their girlfriend. The girl decided to mess around and didn't know it was Aaron. Once she found out she kept texting them.
When Jeremy accidentally pressed the wrong number and texted someone he doesn't know, he never expected it to end in a new friend (even though he doesn't know their name or gender... but is that really necessary information?)
(THIS IS NOT A BE MORE CHILL FANFICTION. I know a lot of people think it is, but Jeremy is my own character, as are all of the rest of them.)